As a method for preparing a ferroelectric film, there is generally known a method in which an alkoxide or organic acid salt of individual constituent metals is dissolved in a polar solvent to prepare a mixed solution, and the resulting solution is applied and dried on a metal substrate to form a coated film, followed by baking at a temperature equal to or higher than its crystallization temperature, thereby forming a dielectric thin film (for example, see Patent Document 1).
Further, for the purpose of use in DRAMs or nonvolatile memories, there is known a method of producing a dielectric device which includes forming an amorphous or crystalline dielectric film on a semiconductor substrate, and doping impurities on the dielectric film by a thermal diffusion method, an ion implantation method, an ion doping method, or the like (for example, see Patent Document 2). The Patent Document 2 discloses a PZT film as a metal dielectric film, and P (phosphorus) ions as impurities. Doping of P (phosphorus) can improve memory retention characteristics of DRAMs or nonvolatile memories equipped with a dielectric capacitor.
Further, for the purpose of use in a capacitor of semiconductor memory cells, it has been disclosed a method of forming a ferroelectric film in which, upon forming a ferroelectric film of PZT by a sol-gel method, a lead-titanium double alkoxide or lead-zirconium double alkoxide is formed, the reaction product is subjected to hydrolysis and a condensation reaction to make a high-molecular weight version, a raw material solution is prepared, the raw material solution is applied, the applied raw material solution is dried to form a dried film, and the dried film is baked (for example, see Patent Document 3). The Patent Document 3 describes that, in order to suppress fatigue (decrease in residual polarization value) or leakage current due to the inversion of applied voltage upon the use of the formed PZT thin film, the fourth metal element such as lanthanum, niobium, or iron may be added to the raw material solution. According to Patent Document 3, hydrolysis and condensation reactions of individual double alkoxides uniformly proceed, and PZT thin films formed from this sol-gel solution exhibit satisfactory electrical characteristics such as smooth surface, large amount of residual polarization and small amount of leakage current, so it is possible to meet the required performance.
Further, for the purpose of use in a variety of devices taking advantage of electrical or optical properties, there is known a composition which is used in the formation of a PLZT ferroelectric thin film, wherein the composition is a liquid composition for the formation of a thin film of a mixed composite metal oxide of a composite metal compound A represented by PLZT and a composite metal oxide formed of one or more elements selected from Bi, Si, Pb, Ge, Sn, Al, Ga, In, Mg, Ca, Sr, Ba, V, Nb, Ta, Sc, Y, Ti, Zr, Hf, Cr, Mn, Fe, Co, Ni, Zn, Cd, Li, Na and K, the composition including a solution wherein a compound constituting the metal oxide is dissolved in an organic solvent in such a proportion as to provide the desired metal atom ratio (for example, see Patent Document 4). In the Patent Document 4, the use of such a composition enables the crystallization even at a low temperature of 450° C. or lower, upon the formation of a ferroelectric thin film.
In addition, for the purpose of use in nonvolatile memories, there is a disclosure of a mixed liquid for the formation of a ferroelectric thin film, wherein Ca, Sr, or La is added to PZT (for example, see Patent Document 5).
Further, when a voltage is applied to a thin film of PZT which is a representative ferroelectric substance, the leakage current density increases which is confirmed to consequently result in a dielectric breakdown.
To cope with this problem, an attempt has been made to improve leakage characteristics by the addition of trace elements to a ferroelectric thin film such as PZT film, but the results are still unsatisfactory (for example, see Patent Document 6 and 7).
In addition, an attempt has been made to decrease the leakage current density by increasing the thickness of the ferroelectric film configuration, but such an attempt has the problem of lowering the capacitance.
As countermeasures against the above-mentioned problems, it has been demonstrated that a specific permittivity of a non-doped PZT film can be improved from 400 to 700 by the doping of cerium nitrate at 1 at. % on a PZT film having a film thickness of about 1 μm, but such a specific permittivity is still low, thus being insufficient for practical use (for example, see Non-Patent Document 1).
Further, when a ferroelectric film is configured into a thin film, a sufficient specific permittivity cannot be obtained due to the action of great stress resulting from the restraint of a substrate (for example, see Non-Patent Document 2).
For these reasons, it has been attempted to improve the specific permittivity through the addition of trace elements (for example, see Non-Patent Document 1).
Further, film thickness reduction theoretically leads to an increase in capacitance, so it has been attempted to improve the capacitance through the reduction of the film thickness.
Further, it has been attempted to improve the dielectric strength voltage characteristics by doping PZT with Bi (for example, see Patent Document 6). However, the Patent Document 6 merely exemplifies Bi as a doping element, but there is no working example illustrating the practical application of doping. In addition, there is no measurement of the specific permittivity therein.
Further, it has been described that the addition of acetic acid to a PZT sol-gel liquid improves the stability of a solution in air (for example, see Patent Document 8). There has been described that upon the addition of an organic acid ester to a PZT sol-gel liquid, a PZT film with improved (111) orientation can be obtained taking advantage of lattice information of the underlying Pt(111) film (for example, see Patent Document 9).
However, there is no report showing an improvement in specific permittivity of the PZT film when organic acid is added to a PZT sol-gel liquid.